As shown in FIG. 7, a conventional triplate line inter-layer connection structure is designed to allow a first triplate line in which a first feed substrate (06) provided with a first feeder line (05) and sandwiched between a first dielectric (04a) and a second dielectric (04b) disposed approximately intermediate between a first ground conductor (01) and a second ground conductor (02), and a second triplate line in which a second feed substrate (09) provided with a second feeder line (08) and sandwiched between a fifth dielectric (07a) and a sixth dielectric (07b) is disposed approximately intermediate between the second ground conductor (02) and a third ground conductor (03), to be electromagnetically coupled with each other through a slit (014) formed in the second ground conductor (02) (see a prior art structure in the following Patent Document 1).
Generally, with a view to suppressing a loss in the feeder line, a low-dielectric constant material having a relative permittivity ∈1≈1 is used for the first dielectric (04a), the second dielectric (04b), the fifth dielectric (07a) and the sixth dielectric (07b). Further, with a view to avoiding the occurrence of a higher-order mode in the transmission line at an operating frequency, each of a distance between the first ground conductor (01) and the second ground conductor (02) and a distance between the second ground conductor (02) and the third ground conductor (03) is set to about ⅕ or less of an effective wavelength at the operating frequency (the effective wavelength=free-space wavelength/square root of relative permittivity of dielectric).
Further, as a prerequisite to allowing the first feeder line (05) and the second feeder line (08) to be electromagnetically coupled with each other through the second slit (014) in an adequate manner, it is necessary to configure the second slit (014) to resonate at the operating frequency. Therefore, as shown in FIG. 8, it is necessary that a resonator length L8, i.e., a length of the second slit (014), is set to about ½ of the effective wavelength at the operating frequency, and the second slit (014) is disposed to be located at a position away from each of a connection-side terminal end edge of the first feeder line (05) and a connection-side terminal end edge of the second feeder line (08) by a line length L7 equal to about ¼ of the effective wavelength at the operating frequency. Basically, a width of the second slit (014) is set to about 1/10 of the effective wavelength at the operating frequency.
As above, the resonator length L8 of the second slit (014) is set to about ½ of the effective wavelength at the operating frequency, so that the second slit (014) is operable to resonate at the operating frequency, and the setup position L7 of the second slit (014) away from each of the connection-side terminal end edges of the first feeder line (05) and the second feeder line (08) is set to about ¼ of the effective wavelength at the operating frequency, so that impedance matching dependent on a position the second slit (014) relative to the feeder lines is ensured to allow electromagnetic waves to be transmitted without being reflected.
In a planar array antenna for use in a vehicle-mounted radar and high-speed communications in a millimeter-wave band, it is important to have high-gain/wide-band characteristic and a capability to efficiently transmit received signals from a plurality of antennas to an electromagnetic-wave receiving/transmitting section so as to achieve required angle detection accuracy in a frequency band.
As a planar array antenna designed in view of the above point, the following Patent Document 2 discloses a low-cost planar antenna module which is low in loss and characteristic variation due to assembling errors, and stable in frequency characteristic. A structure of this planar array antenna module is shown in FIG. 5 and FIG. 7 of the Patent Document 2 (FIG. 26 and FIG. 27 of this application)
FIG. 5 of the Patent Document 2 (FIG. 26 of this application) shows an antenna section (101) which comprises an antenna substrate (40) formed with a plurality of antenna arrays each composed of a combination of a first feeder line (42) connected to a radiation element (41), and a first connection portion (43) electromagnetically coupled with a feeder section (the entirety of FIG. 27).
FIG. 7 of the Patent Document 2 (FIG. 27 of this application) shows the feeder section (102) and a second connection portion (52), wherein the first connection portion (43) in FIG. 26 and the second connection portion (52) in FIG. 27 are electromagnetically connected to each other via a second slot (24).